The present invention relates to a scanning receiver which selects and receives a target radio wave out of various radio waves, and particularly to a scanning receiver capable of capturing and receiving a target radio wave without the need for many scanning processes and a long selection time even if the propagation frequency of the target radio wave is unknown.
In general, a scanning receiver is operated so as to frequency-sweep a local oscillation frequency over a constant or predetermined frequency range and stop the sweeping of the local oscillation frequency when a target radio wave is captured and received by its frequency sweeping thereby to proceed to the reception of the target radio wave.
In such a scanning receiver, the sweeping of the local oscillation frequency has originally been performed using an analog means upon frequency-sweeping the local oscillation frequency. Therefore, when a firstly captured and received radio wave does not correspond to a target radio wave, the remaining frequency range is frequency-swept following its capturing to thereby capture the target radio wave. In such a case, information about the local oscillation frequency at the stop of the frequency sweeping cannot be stored perfectly. It was thus difficult to perform frequency sweeping over a wide band frequency range and thereby capture and receive a target radio wave.
Thereafter, with rapid development of digital technology substituted for analog technology, the digital technology has been used in the frequency sweeping of the local oscillation frequency even in the field of scanning receivers. Thus, a scanning receiver that makes use of a digital memory means has become developed.
In recent years, computer processing technology and LSI (Large Scale Integration) manufacturing technology have greatly been advanced and global homogenization in the allocation of frequencies of radio waves has been made. Therefore, receivers each using a PLL (Phase Locked Loop) frequency synthesizer in the generation of a local oscillation signal have increased. Even in the case of a scanning receiver, one using such a PLL frequency synthesizer has become mainstream.
On the other hand, the scanning receiver needs to take into consideration the following points when the PLL frequency synthesizer is used in the generation of the local oscillation signal. That is, when the propagation frequency of a target radio wave is perfectly unknown, the scanning receiver must scan received radio waves over an extremely wide frequency range to capture and receive the target radio wave in order to capture and receive the corresponding target radio wave. The reason for this is as follows: When the frequency allocations of actual various radio waves are viewed, they are given at alternate-12.5 kHz frequency intervals or at alternate-25 kHz frequency intervals depending upon frequency bands (such allocated positions of radio waves hereinafter be called “channels”). Therefore, in order to capture and receive the target radio wave by the scanning receiver, there is a need to sequentially capture and receive the respective channels at above frequency intervals over the full frequency range in which effective radio waves are placed, regardless of the presence or absence of allocation of the radio waves.
In this case, the scanning receiver needs to frequency-change the local oscillation frequency generated by the PLL frequency synthesizer in such a manner that the local oscillation frequency is sequentially synchronized with the propagation frequencies of the radio waves placed at above frequency intervals. Since a frequency switching time of about 10 ms per channel is normally required where the local oscillation frequency of the PLL frequency synthesizer is frequency-changed so as to be synchronized with the propagation frequencies of the respective radio waves, a considerable time might be required until the wide frequency range in which the effective radio waves exist, is frequency-swept to capture and receive the target radio wave.
Even in the case where it is expected that, for example, a transmitting station transmitting a target radio wave is at a fairly near distance and the field intensity of the target radio wave will be relatively large, the need to take a considerable time is also by no means rare until the target radio wave is captured and received.
In consideration of the fact that time is extremely taken to capture and receive such a target radio wave, the shortening of the time taken to capture the target radio wave has been thought out in the field of a sweeping receiver containing this type of scanning receiver. A receiver disclosed in a patent document 1 (Japanese Unexamined Patent Publication No. Hei 09(1997)-321581) has been proposed as one example.
The receiver disclosed in Japanese Unexamined Patent Publication No. Hei 09(1997)-321581 is one of such a type that two receiving sections corresponding to a main receiving section and a sub receiving section configured substantially in the same scale are provided, and while any receiving radio wave is being received by the receiving operation of the main receiving section, the sub receiving unit performs a sweeping operation to detect the following received radio wave, and data about the detected received radio wave is stored in a memory, and that when a skip signal is supplied to the main receiving section, the main receiving section receives the next received radio wave on the basis of the received radio wave data stored in the memory. Thus, since the main receiving section is capable of receiving the following radio wave promptly even if the speed of frequency sweeping at the sub receiving section is not so fast, the frequency sweeping operation can apparently be speeded up looking overall.
Although the receiver disclosed in Japanese Unexamined Patent Publication No. Hei 09(1997)-321581 is capable of apparently speeding up the frequency sweeping operation by cooperation between the main receiving section and the sub receiving section and thereby shortening time taken to capture the target radio wave, it is necessary to provide the sub receiving section substantially identical in constitutional scale to the main receiving section in addition to the provision of the originally-required main receiving section. Therefore, the receiver becomes large in constitutional scale on the whole but also increases in the number of required components, thereby resulting in a great increase in manufacturing cost.